Spectrometry using continuous wave (CW) tunable sources with narrow spectral linewidth and long coherence lengths has well-known advantages associated with high spectral contrast, frequency selectivity and excellent sensitivity. Scanning CW terahertz (THz) spectrometers are a prime example of this technology. A negative consequence of long coherence lengths, however, may be the transmission modulation resulting from reflections from spurious resonant cavities. A significant and often dominant noise source may be the random amplitude variation resulting from source frequency random jitter and repeatability error coupled with the standing wave transmission modulation (intensity gradients) of these spurious cavities.
Spurious cavities may be formed by Fresnel reflections from any surfaces between the transmitter and receiver electromagnetic wave propagation path. The reflectivity of these surfaces may be difficult to suppress because of the wide frequency range covered in the THz spectrometer making anti-reflective (AR) coatings ineffective. It would therefore be desirable to design an apparatus and method of effectively suppressing noise due to random amplitude variations from spurious cavities.